1. Technical Field
The present invention relates to a molded commutator of a motor used for a cleaner and a power tool, and also relates to a method of manufacturing the molded commutator.
2. Background Art
In a motor employed for a domestic vacuum cleaner, because of its rotation at a high speed, each of the commutator segments of a molded commutator undergoes a centrifugal force, by which a step is formed on the surface of the molded commutator. A large step can accelerate mechanical wear of a brush or cause unwanted spark discharge (hereinafter referred to as “spark”) between the brush and the commutator segments. It is therefore important that such a step should be as small as possible. To address the problem above, for example, Japanese Examined Patent Application Publication No. H05-38544 introduces a molded commutator where column-shaped ceramic is provided between the commutator segments to decrease the step to as small as several micrometers.
On the other hand, in a low-voltage DC motor used for information equipment, a varistor is externally disposed between the commutator segments. When voltage exceeds a predetermined level, the varistor has a steep decrease in resistance values to increase current flow. The characteristics of the varistor suppress spark voltage between the commutator segments, contributing to an extended operating life of the brush and a noise-reduced motor. For example, Japanese Unexamined Patent Application Publication No. H06-204006 discloses a method of manufacturing a zinc-oxide varistor suitable for low-voltage driving.
The molded commutator used for the low-voltage DC motor above, however, has many commutator segments; fixing varistors between the commutator segments is time-consuming work. Besides, the varistors should be protected from the centrifugal force generated by rotation of the motor. Furthermore, in a motor used for a vacuum cleaner, high voltage is applied between the commutator segments; a varistor having a low varistor voltage cannot be used in such a motor.